This work contributes to the estimation of new and complementary density data for carbon dioxide (CO2) confined in carbon slit pores at different conditions. Grand canonical Monte Carlo (GCMC) simulations were employed to predict the CO2 adsorption capacities in carbon slit pores of height 20, 31.6, 63.2, 94.85 and 126.5 Å at 673.15 and 873.15 K over a pressure range of 500–4000 kPa, which corresponds to steam reforming of methane process. The bulk densities of CO2 at these temperature and pressure conditions have been estimated via isothermal–isobaric ensemble MC simulations using the Elementary Physical Model. The predicted density shows an excellent agreement with the experimental data. The adsorption capacities of CO2 in all the aforementioned pores at 673.15 and 873.15 K over the pressure range of 500–4000 kPa have also been estimated in the presence of wall–fluid interactions, in addition to the fluid–fluid interactions. The study on the thermodynamic phase behaviour of confined CO2 in the presence of wall–fluid interactions showed the existence of vapour–liquid equilibria at high temperature and pressure conditions. © 2020, Indian Academy of Sciences.